1. Field of the Invention
The present invention relates to a method for preparing aluminum nitride. More particularly, it relates to a method for preparing high-purity aluminum nitride powder which is used suitably for aluminum nitride substrates and the like and in which oxygen and carbon contents are low, and it also relates to a sinter of the aluminum nitride powder.
2. Description of the Prior Art
Recently, in the field of microelectronics, the trend is toward increases integration and output power increase and, for this reason, alumina substrates which have been heretofore used are already insufficient as heat release substrates on which semiconductors are mounted. In consequence, much attention is paid to aluminum nitride substrates as new heat release substrates having high thermal conductivity and heat resistance.
Conventional and known methods for preparing aluminum nitride powder for use in the aluminum nitride substrates are as follows:
1. A method which comprises heating metallic aluminum in nitrogen or ammonia atmosphere.
2. A method which comprises mixing alumina powder with carbon powder, and then heating the mixture in a nitrogen or ammonia atmosphere.
3. A method which comprises thermally treating the reaction product of an organic aluminum and an amine.
In the case of the above-mentioned method (1), the melting point of aluminum and a nitrogenization temperature are close to each other, and therefore the product obtained thereby is solidified. Thus, a process for grinding the product is additionally necessary, and during this grinding process, the product is inconveniently susceptible to being contaminated with impurities such as oxygen. Furthermore, since it is impossible to remove the impurities from raw material aluminum, the high-purity products are hardly manufactured. In the case of the alumina reduction process (2), impurities in the alumina and also coarse grains of alumina, if the latter is used as raw material, undesirably remain in the product. Thus, it is necessary that the raw material alumina has a high purity and is finely ground, which increases material costs. In addition, unreacted carbon is left in the product, and when the unreacted carbon is removed by oxidation, the aluminum nitride product is also partially oxidized disadvantageously.
In contradistinction, in method (3) the organic aluminum starting material is usually liquid, and a high-purity product is available with ease by distillation or the like. Thus, this method can prepare relatively high-purity aluminum nitride.
In fact, however, this method involves various other problems and hence it is not yet in commercial use. For example, Japanese laid open Patent Publication No. 1978-68700 discloses a method which comprises first reacting an organic aluminum compound with ammonia or a primary or secondary amine to form an aluminum nitride precursor, and heating the precursor at a temperature of 400.degree. C. or more in an inert gas, in vacuo or an ammonia gas flow. However, this method has the drawback that a large amount of carbon remains in the thus-produced aluminum nitride and, what is worse, the remaining carbon cannot be removed therefrom. Japanese laid open Patent Publication No. 1987-108720 discloses a method for preparing aluminum nitride which comprises first reacting a high-purity organic aluminum compound with an organic amine or hydradine in order to form an aluminum nitride precursor, and then carrying out primary baking in a non-oxidizing gas flow and secondary calcining at 1,150.degree. C. or more in a hydrogen gas flow. According to this method, the much carbon-containing aluminum nitride which has been prepared by about the same method as in the above-mentioned Japanese laid open Patent Publication No. 1978-68700 is calcined secondarily in a reducing gas flow to remove carbon therefrom. However, in this method, hydrogen is used at a temperature of 1,150.degree. C. or more, and therefore the method is not considered to be safe. Additionally, in this method, the calcination is carried out twice at the high temperature, which is not economical.
The problems associated with the above-mentioned method (3), including those described above, can be summarized as follows:
(a) In the produced aluminum nitride, carbon remains in large quantities.
(b) In order to remove the residual carbon, heating must be carried out at 1,150.degree. C. in hydrogen, which is very dangerous.
(c) When an after-treatment such as an air oxidation treatment is additionally used in order to remove the residual carbon, the amount of oxygen in the thus-obtained aluminum nitride increases.
(d) Fine particles cannot be manufactured with good controllability.
There therefore is a need for a method for simply and safely preparing fine-particle aluminum nitride containing less carbon and oxygen.